1. Field of the Invention
This invention relates to telephoto zoom lenses and, more particularly, to telephoto zoom lenses having four lens groups of which the first three counting from the front are movable to effect a change in image magnification, with the total length of the lens system being shortened.
2. Description of the Prior Art
In the past, as one type of zoom lens, there has been known a lens system consisting of four lens groups having positive, negative, positive and positive refractive powers arranged in this order from the front. In such zoom lens the 2nd lens group is moved axially to effect variation in the focal length, while the image shift compensation is effected by the axial movement of the 3rd lens group in a path concave toward the front. However, generally in this type of zoom lens, an increase in the zoom ratio or the relative aperture calls for a rapid increase of the total length of the lens system or the diameter of the front lens members. Further, to maintain the constant total length of the lens system during zooming, a large limitation must be given to the ratio of the distance from the front vertex to the focal plane to the longest focal length, that is, the telephoto ratio. Also, to suppress this to a minimum, the refractive power of each of the lens group in the varifocal section must be strengthened, thereby producing disadvantages that the necessary number of constituent lens elements is increased and good correction of principal aberrations becomes difficult to perform. On the other hand, it is known to provide zoom lenses of which the varifocal section includes three or more lens groups as in Japanese Laid-Open Patent No. Sho 53-34539, and U.S. Pat. Nos. 4,172,635, 4,196,969 and 4,240,760. These zoom lenses must however be provided with their operating mechanisms of complex structure, and such configuration cannot always be said to be suitable for use in the telephoto type zoom lens.